Recently, automobile intermediate and top coatings frequently employ high solid coating compositions for reducing the solvent and assuring a reduction in solvent release in the coatings.
Particularly in the United States, there is a rigorous control by the Environmental Protection Agency which bans the use of automotive coatings other than high solid systems or the equivalent thereof to reduce such solvent release phenomenon.
One of the problems associated with the use of such a high solid finish coating is the difficulty of matching colors in repairing the coatings. This is particularly true when attempting to repair a metallic coating film which has been damaged.
When the car is on the production line of a car manufacturer where it has not yet been fitted with components, interior fittings, etc., it is possible to repair any coating defect with a production coating enamel and bake it on at a high temperature (usually higher than 140.degree. C.). Inasmuch as the same enamel as the production enamel is used for repairing in this manner, there is no remarkable difficulty in matching colors. However, when the fully assembled car is found to have a defective or marred coat after rolling off the production line or in the marketing place, there are several problems involved in repairing the defects with the same baked-on type enamel as used in the production enamel.
Since the repairing process involves application of a high temperature of about 140.degree. C., the component parts and internal fittings vulnerable to such high temperature must be removed in the first place. This dismounting operation is time-consuming and costly.
The use of a touch-up enamel of a cold cure type or room temperature drying type, dissimilar to the production enamel, for repairing would dispense with the tedious dismounting operation but this should present the following two problems.
One of the problems is that it is difficult to obtain a film performance equivalent to the original coating film. In order to assure the same shell coat film performance, it is necessary to select and use an appropriate coating system such as a two-package urethane coating composition.
The second problem is that because such a touch-up coating differs from the original enamel in its cure and drying mechanisms, it is difficult to achieve exact color matching.
Particularly in the case of a metallic enamel, even the use of the same aluminum pigment coating system tends to produce a different shade due to different topological distribution and orientations of aluminum owing to the different cure mechanisms and solvent evaporation rates involved. Furthermore, even if color matching is satisfactory when viewed from one direction, the phenomenon of so-called metamery occurs when viewed from a different direction.
As a means for globally overcoming the above two problems related to color matching and dismounting of parts and fittings, it has been proposed to modify the baked-on enamel system currently used in the production line by some means or other so that no high temperature steps are needed. The reduced temperature, as such, may be as the rule of thumb, 80.degree. C. at which removal of the parts and fittings will not be necessary.
To approach this goal, it has been proposed to add an excess of a melamine compound or add an isocyanate compound to the system to cause it to react with the unreacted-OH groups.
However, in the case of a high solid melamine-acrylic resin coating system, since its formulation includes a low molecular acrylic resin, a low molecular melamine resin and an acid catalyst, the above proposals do not assure the necessary film performance because no sufficient crosslinking reaction takes place at said reduced temperature.